1. Field of the Invention
The present invention relates to a variable resistor formed on a silicon substrate, a filter, a variable gain amplifier and an integrated circuit using the variable resistor.
2. Description of the Related Art
Conventionally, there has been known the following variable resistor. The variable resistor is composed of common resistors, a plurality of tuning resistors and switches. The tuning resistors connected to the common resistor are switched via the foregoing switches, and thereby, a desired resistance value is obtained.
When a variable resistor is formed on a silicon substrate, a parasitic capacitance occurs between the substrate and the variable resistor. The parasitic capacitance between the substrate and the variable resistor is a factor of causing a signal phase delay. In addition, when the tuning resistors are switched, a voltage division ratio of the common resistor and the tuning resistors changes. For this reason, voltage amplitude of a node between the common resistors and the tuning resistors is not constant. Thus, an influence of a parasitic capacitance between the node and the substrate is not constant; as a result, fluctuation occurs in the signal phase delay. For example, if a filter is configured using the foregoing variable resistor, a filter characteristic is deteriorated due to the foregoing fluctuation of signal phase delay.
The following document describes a Quality-factor tuning circuit to control the foregoing fluctuation of signal phase delay.
Document: S. Kousai et al, “A 19.7 MHz, 5th Oder Active-RC Chebyshev LPF for IEEE 802.11n with Automatic Quality Factor Tuning Scheme,” A-SSCC 2006
When the conventional variable resistor is formed on the silicon substrate, following problems occur. Specifically, a signal phase delay occurs due to a parasitic capacitance and fluctuation of the phase delay occurs. If the phase delay is constant, the phase delay is canceled using a circuit technique. However, a complicated tuning circuit such as a Q tuning circuit is required to control the fluctuation of phase delay. As a result, this is a factor that leads to an increase in the circuit complexity.